xDSL is a generic term for various types of digital subscriber lines (DSL), and is a high speed data transmission technology implemented by using unshielded twist pairs (UTP). An xDSL technology for passband transmission uses a frequency division multiplexing technology to enable an xDSL service and a plain old telephone service (POTS) to co-exist on a same twisted pair, where the xDSL service occupies a high frequency band, the POTS occupies a baseband part below 4 kHz, and POTS signals are separated from xDSL signals by a splitter. The xDSL technology for passband transmission uses discrete multi-tone (DMT) modulation. A system providing access for multiple xDSLs is called a digital subscriber line access multiplexer (DSLAM). According to an electromagnetic induction principle, interference, which is called crosstalk, is generated between multiple channels of signals that are received by the DSLAM. Energy of both near end crosstalk (NEXT) and far end crosstalk (FEXT) enhances with a rise in a frequency band. xDSL upstream and downstream channels use frequency division multiplexing, and the NEXT has a smaller impact on performance of a system. However, as the xDSL uses an increasingly wide frequency band, the FEXT affects line transmission performance more seriously. All systems for undistorted communication follow a famous Shannon's equation: C=B·(1+S/N), where C is channel capacity, B is a signal bandwidth, S is signal energy, and N is noise energy. In xDSL transmission, crosstalk is represented as part of noise, so serious FEXT significantly reduces a channel rate. When multiple users from a bundle of cables require provisioning of an xDSL service, the FEXT causes some lines to have low rates and unstable performance and even to encounter cases such as a failure to provision the service, and eventually leads to a low line activation ratio of the DSLAM. At present, the industry puts forward a vectored-DSL (Vectored-DSL) technology, which mainly makes use of possibility of performing coordinated reception and transmission at a DSLAM end to use a signal processing method to cancel interference of the FEXT. For downstream transmission, a pre-coder P is introduced at the DSLAM end to perform coordinated transmission of signals, and the signals are separately received at user ends; for upstream transmission, signals are separately transmitted at the user ends, and a crosstalk canceller W is introduced at the DSLAM end to perform coordinated reception of the signals to cancel crosstalk. In this process, synchronization is performed by using synchronization symbols (Sync Symbol). A vectoring control entity (VCE) uniformly allocates a pilot sequence to all lines, transceivers (transceiver) on the lines jointly modulate, on the sync symbols, the pilot sequence allocated by the VCE, and finally the VCE receives clipped errors fed back by a corresponding customer premises equipment (CPE) or CO-end transceiver, so that a downstream precoding matrix P′ and an upstream cancellation matrix W′ can be estimated in the VCE according to the pilot sequence and the clipped errors, and ultimately the FEXT is eliminated. Functions of the downstream precoding matrix P′ and the upstream cancellation matrix W′ are usually implemented in a canceller.
However, a second generation very-high-bit-rate digital subscriber loop (VDSL2) technology is earlier than the Vectored-DSL technology, and the VDSL2 technology has been widely used; therefore, compatibility with a VDSL2 legacy CPE that exists on a live network and does not support Vectored-DSL needs to be considered when VDSL2 is upgraded to the Vectored-DSL. However, the VDSL2 legacy CPE does not support transmission and reception of a pilot sequence or feedback of clipped errors on sync symbols; as a result, it is difficult for the VCE to estimate upstream and downstream cancellation matrixes that are used to cancel crosstalk coming from a legacy line to a Vectored line. When a vectored line in a data transmission time (Showtime) state exists in a system, as a Legacy line is added to the system, in a case in which crosstalk coming from the Legacy line is not canceled, error codes increase on the Vectored line in the Showtime state due to a decrease of a signal-to-noise ratio (SNR), which seriously affects a rate of the Vectored line and stability of an entire Vectored-DSL system.
In the prior art, all VDSL2 Legacy CPEs on a live VDSL2 network can be upgraded or changed to VDSL2 Vectored CPEs to solve problems of incompatibility between the VDSL2 Legacy CPEs and the VDSL2 Vectored CPEs and a failure to eliminate FEXT. However, upgrading all the Legacy CPEs on the live network needs to consume a lot of costs, and some earlier Legacy CPEs may fail to be upgraded to the Vectored CPEs due to various causes, for example, error calculation and error feedback are not supported, so that an entire CPE needs to be replaced, which further leads to an increase in costs.